The present invention relates to a high-speed diode having a small reverse recovery current and a device produced by application of the diode. In particular, it relates to a semiconductor device and a method of producing the same, in which it is possible to make a diode high in withstanding voltage, high in reliability and simple in manufacturing.
FIG. 9 shows current and voltage waveforms of a general diode in which the state thereof changes from a current conductive state in the forward direction (hereinafter referred to as "forward current-conductive state") into a current blocking state in the reverse direction (hereinafter referred to as "reverse current-blocking state"). When a current with a current density J.sub.F is passed in the forward direction and then a voltage V.sub.R is instantaneously applied in the reverse direction, a reverse recovery current flows. It is necessary to reduce the peak value of the reverse recovery current density J.sub.RP at this time to be as small as possible, because a power loss is generated proportionally to the peak value of the current density J.sub.RP. Further, the peak value of the current density J.sub.RP acting as a noise source becomes a cause of a faulty operation in a circuit using the diode, in particular, an integrated circuit using the diode. From this point of view, a diode structure for reducing the current density J.sub.RP as shown in FIG. 10 has been discussed in the papers of IEEE International Electron Devices Meeting, pages 658-661, 1987. In this structure, a p layer 113 separated into parts is formed in an n- layer 112 which is formed on one surface of an n+ substrate 111 by a technique such as a crystal growing technique. An electrode 121 is disposed so as to be in ohmic contact with the p layer 113 and forms a Schottky junction with exposed portions of the n- layer 113 where the p layer 113 is not formed, that is, exposed portions of the n- layer 112 which are respectively disposed between the separation parts of the p layer 113. The electrode 121 is formed so as to extend onto an oxidized film 131 in the peripheral portions thereof, so that the electrode 121 serves as a field plate for relaxing the electric field in the peripheral portions thereof. An opposite electrode 122 is disposed so as to be in low ohmic contact with the n+ layer 111. When a current is passed through the diode from the electrode 121 to the electrode 122, holes are injected through the pn junction portions, that is, from the p layer 113 to the n- layer 112, so that excess carriers are accumulated in the n- layer 112. However, holes are little injected through the Schottky junction portions from the electrode 121 to the n- layer 112. Accordingly, the concentration of carriers accumulated in the vicinity of the interface between the pn junction and the Schottky junction is reduced compared with the conventional diode having only pn junctions. Consequently, as is obvious from FIG. 9, the diode of FIG. 10 has an advantage in that it is effective for reduction of the current density J.sub.RP, because the current density J.sub.RP at the instance when the reverse bias V.sub.R is applied is produced by the carriers accumulated in the vicinity of the pn junctions. Furthermore, in a reverse current-blocking state, because a depletion layer extending from the pn junctions which are formed between the p layer 113 and the n- layer 112 and disposed on the both sides of the Schottky junction reaches through under the Schottky junction so that the electric field applied to the Schottky junction can be relaxed. Accordingly, the diode has another advantage in that a leakage current can be reduced compared with the conventional diode having only Schottky junctions.
On the other hand, a diode structure for reducing the current density J.sub.RP as shown in FIG. 11 has been disclosed in Japanese Patent Unexamined Publication No. Sho-58-60577. The diode of FIG. 11 is different from the diode of FIG. 10 in that a p layer 114 having a carrier concentration lower than that of the p+ layer 113 is provided on the exposed surface portions of the n- layer 112 which are located between separated parts of the p+ layer 113. The electrode 121 is disposed so as to be in ohmic contact with the P+ layer 113 and the P layer 114. Accordingly, because a current is mainly passed through the pn junctions between the p layer 114 and the n- layer 112 which is small in diffused potential when the diode is in a forward current-conductive state, the diode has an advantage in that a forward voltage drop can be reduced compared with the diode having only the pn junctions between the p+ layer 113 and the n- layer 112. Furthermore, because the carrier concentration in the p layer 114 is low, the quantity of carriers injected from the p layer 114 can be reduced. Accordingly, the diode has another advantage in that the current density J.sub.RP can be reduced. Furthermore, because a metal-semiconductor interface such as a Schottky junction is not used, the diode is little affected by factors such as contamination at the semiconductor surface. Accordingly, the diode has a further advantage in that the diode has stable characteristics. Of course, the diode of FIG. 11 has the same effect as the diode of FIG. 10 in that the electric field applied to the pn junctions between the p layer 114 and the n- layer 112 can be relaxed by the depletion layer extending from both the deep p+ layer 113 and the n- layer 112 to thereby reduce a leakage current.